1. Field of the Invention
The present invention relates to a flow rate measuring apparatus for measuring a flow rate of a fluid, and particularly to a flow rate measuring apparatus suitable for measuring the flow rate of intake air sucked into an internal combustion engine of an automobile.
2. Description of the Related Art
Conventional flow rate measuring apparatuses are configured by mounting a mold in which a half cross-section of an auxiliary air passage is formed to a modular housing in which a half cross-section of the auxiliary air passage is formed. The half cross-section of the auxiliary air passage of the modular housing and the half cross-section of the auxiliary air passage of the mold function together to constitute an auxiliary air passage. The modular housing is formed by fixing an L-shaped metal base, terminals, and a lead frame by plastic molding. At that time, a circuit board housing portion, a connector, and the half cross-section of the auxiliary air passage are formed in the modular housing simultaneously. The circuit board is fixed onto the metal base inside the circuit board housing portion, and the terminals, the lead frame, and the circuit board are electrically connected. In addition, a heating coil functioning as a flow rate detecting element has a first end connected to the lead frame and is disposed so as to extend inside the auxiliary air passage. These conventional flow rate measuring apparatuses are inserted such that the auxiliary air passage is positioned inside a main air passage, and are mounted by screwing the metal base onto the main air passage. (See Patent Literature 1, for example.)
Patent Literature 1: Japanese Patent Laid-Open No. HEI 9-4487
(Gazette)
When a conventional flow rate measuring apparatus is used in an engine fuel injection system of an automotive internal combustion engine, the flow rate measuring apparatus is mounted by screwing the metal base onto an air intake passage of the automotive internal combustion engine. In the case of this air intake passage of the automotive internal combustion engine, ambient temperature is low, and the air intake passage is cold, but the air intake passage itself may also be subjected to the influence of heat from the internal combustion engine and become warm. Since a portion of the metal base constitutes the auxiliary air passage, heat from the air intake passage is conducted through the metal base and transferred to the air flowing through the auxiliary air passage. Thus, air temperature in a vicinity of the heating coils projecting into the auxiliary air passage becomes high, and becomes a factor generating flow rate detection errors.
In conventional flow rate measuring apparatuses, because a heating coil functioning as a flow rate detecting element has a first end connected to the lead frame and is disposed so as to extend inside the auxiliary air passage, misalignments in the relative position of the heating coil and the auxiliary air passage arise easily, and one problem has been that relationships of output relative to flow rate (hereinafter called “flow rate measurement characteristics”) becomes irregular among flow rate measuring apparatuses.